This invention relates to a method for compensating measured concentration values of at least a first chemical compound within a first internal tract of a patient, which concentration values as measured are affected by a second chemical compound, which is becoming present in the body of said patient and has a limited transfer speed to mix with the first chemical compound at least in a detection site for said measured concentration values, rendering these measured concentration values to deviate from actual concentration values within said first internal tract. Especially invention relates to a use of the method for compensating measured concentration values of carbon dioxide within an gastrointestinal tract of a patient, which concentration values are affected by at least nitrous oxide, which is being dosed into the respiratory gas of said patient and has a limited absorption and/or diffusion speed to a detection site for said carbon dioxide concentration measurement, rendering its measured concentration values to deviate from actual concentration values.
For example gastric tonometry provides information on the regional partial pressure of carbon dioxide, which relates to the adequacy of perfusion and rate of metabolism of the patient. The tonometric measurement is done in hollow viscus, typically gastrointestinal, by a special catheter, which is inserted nasogastricly into the stomach or gut. The catheter tip includes a gas-permeable and liquid-impermeable balloon. Carbon dioxide present or developing freely equilibrates between the gastric mucosa, the gastric lumen, and the gas contents of the balloon. After an equilibrium time the catheter gas content corresponds to the gas or liquid composition in the gastric mucosa. Gastric tonometric monitoring apparatus may be designed to automatically infuse and sample gas mixture from the catheter balloon at intervals and then automatically provide data of the carbon dioxide (CO.sub.2) concentration of the gas sample e.g. with the infrared absorption technique. Gastric tonometry is currently used mainly for intensive care patients, but it is increasingly used also for surgical patients during longer operations.
The hepatosplanchic area, which is the usual site of tonometric measurement, has an important role in the pathogenesis of the multiple organ dysfunction syndrome. Splanchnic tissue perfusion has a low priority in the acute circulatory failure. Blood circulation is redistributed during a hypovolemic or cardiogenic shock so, that the most vital organs, heart and brain, get enough blood. Even though the reduction of splachnic tissue perfusion may initially be vital for survival, the prolonged reduction of splanchnic blood increases the risk of tissue damage and organ dysfunction and failures and even death of the patient. The medical background of this phenomenon is disclosed in a booklet published by the applicant: Jukka Takala--"Clinical Application Guide of Gastrointestinal Tonometry", .COPYRGT. Instrumentarium Corp., Datex-Ohmeda Division, Finland (894796-1/PG5/0898) and Guillermo Gutierrez, Steven D. Brown, "Gastric Tonometry: A new Monitoring Modality in the Intensive Care Unit", J Intensive Care Med, 10, pages 34-44, 1995. The inadequate tissue perfusion and/or increased metabolic rate can be seen as an increased partial pressure of carbon dioxide (P.sub.CO2) in the tissue. During inadequate perfusion tissue is not oxygenated well enough, which leads to the anaerobic metabolism and thus increased production of carbon dioxide. Carbon dioxide level in the tissue is also increasing, because it cannot be removed effectively from the tissue. If the increased P.sub.CO2 level can be detected early enough, the patient can be recuscitated better. This problem is discussed in the publication WO-94/21163 and a method is suggested, according to which the partial pressure of carbon dioxide is measured in hollow viscus and the carbon dioxide level or the pH of arterial blood is measured as well, and a parameter, more detailed called either a pCO.sub.2 -gap or a pH-gap, indicative of condition of the hollow viscus is determined on the basis of these two values. The carbon dioxide level of the arterial blood might also be measured through detection of the end tidal partial pressure (pCO.sub.2) of carbon dioxide to describe the overall "global" or "systemic" bicarbonate content of the blood. The pH-gap is calculated by mathematically subtracting intramucosal pH from arterial pH, but calculating pCO.sub.2 -gap is not clearly described. The additional detection and independent display of anaesthetic gases, such as N.sub.2 O, in the aspirated air of the patient using separate techniques is mentioned, which is very elaborate method and requires additional equipment.
The partial pressure (P.sub.CO2) of carbon dioxide level in the gastrointestinal area is determined by analysing the carbon dioxide (CO.sub.2) concentration sampled from the tonometric catheter balloon. The measurement/detection technique for determining this partial pressure (P.sub.CO2) of carbon dioxide is utilises an infrared absorption sensor. An infrared sensor typically comprises an infrared radiation source, gas measuring chamber, at least one optical bandpass filter within an absorption peak of CO.sub.2 and an infrared detector delivering an electrical signal proportional to the amount of carbon dioxide in the measuring chamber. The purpose of the optical bandpass filter, like interference filter, is to choose the wavelengths, where CO.sub.2 molecules absorb infrared (IR) light. This kind of measuring sensors and measuring apparatuses are widely known and are generally available in the market by several manufacturers and used for many different measuring purposes. So it is not necessary to describe them more detailed. Nitrous oxide (N.sub.2 O) is commonly given to surgical patients in very high concentrations (20-80%) as an anaesthetic gas in the respiratory tract. A typical inhaled gas mixture includes in the order of 60-70 vol.-% of N.sub.2 O and in the order of 25-30 vol.-% oxygen, and further containing an additional vaporised anaesthetic agent as halothane, desflurane, isoflurane, enflurane and/or sevoflurane usually less than about 12 vol-%. There is known a special measurement error caused by N.sub.2 O to CO.sub.2 because of spectral line broadening, also called collision broadening. This error depends on the CO.sub.2 sensor, but typically this error can be about +10% (relative) with 70 vol.-% N.sub.2 O concentration. So if the true gas mixture contains 10 vol.-% CO.sub.2 and 70% of N.sub.2 O and balanced nitrogen (N.sub.2), an uncorrected infrared absorption sensor described above shows a value 11 vol.-% carbon dioxide. This error is normally not acceptable in tonometric measurement and should be somehow corrected. This problem is typical when measuring with an optical bandpass filter having a narrow transmission band, which extends across several rotational lines of an absorption peak. In this case a detector "sees" an increased absorption value, due to the fact that this kind of measuring arrangement detects transmission instead of actual absorbance. Especially polar gases, N.sub.2 O being one of them, have a major effect on spectral line broadening. It is also possible that e.g. intravenous dosage of drugs or medicinal preparations or additives into the patient may affect the concentration measurements of a chemical compound in an internal tract or organ, either because of spectral line broadening or an overlapping absorption described later.
One way of compensating the line broadening error caused by N.sub.2 O is to use a double detector combination as a sensor arrangement, which detectors simultaneously measure both the CO.sub.2 and the N.sub.2 O concentrations in the same gas mixture. This measurement can be done by positioning two optical filters, which have different radiation transmitting bands, one for CO.sub.2 and one for N.sub.2 O, to detect IR-radiation absorption in a measuring chamber. When both concentrations in the same gas mixture are thus known, a linear mathematical correction model can be applied to CO.sub.2 concentration. This kind of sensor arrangement and correction method is disclosed in publication U.S. Pat. No. 4,423,739 to correct the carbon dioxide measurement result of exhaled air using the nitrous oxide measurement result of the same exhaled air. Both measurements are performed simultaneously at the end tidal of the patient's exhaled breath. The publication teaches a formula for said correction of CO.sub.2 in presence of N.sub.2 O as follows: EQU (C.sub.CO2).sub.breath =(E.sub.CO2).sub.breath [1+K.multidot.(E.sub.N2O).sub.breath] (1),
where C.sub.CO2 =corrected CO.sub.2 concentration in exhaled air, E.sub.CO2 =measured CO.sub.2 concentration in exhaled air, E.sub.N2O =measured N.sub.2 O concentration in exhaled air, and K is an empirical spectral line broadening constant. This disclosed method and apparatus have several drawbacks. For example each gas component, which might be present and might contribute to collision broadening must be measured separately, which is unpractical, because at least individual optical bandpass filter(s) and measuring channels are required for each gas component rendering the large and too heavy sensor, and unreliable, because the gas mixture may include varying amounts of hard-to-measure gas components.
Another publication EP-0 834 733 discloses a totally different method for correcting the spectral line broadening error, with utilising a measured viscosity or viscosity related quantity of the gas mixture. This method also requires an use of additional detection means, and hence are impractical in many cases, where a small and light-weight detector arrangement is needed or is preferable.
An additional problem, independent of said spectral line broadening, is the generally known overlapping absorption of different gas components in a gas mixture, which means that these gas components have absorption peaks very close to each other, whereupon it is difficult to separate the transmissions or absorptions thereof, because each signal from detectors provided with different optical bandpass filters--and intended for a specific gas component--includes transmission/absorption data from at least one other gas component. One suggestion for solving this problem is disclosed e.g. in publication U.S. Pat. No. 4,914,719, according to which the same amount of signals, each gained by an optical bandpass filter with a center wavelength different from the center wavelengths of the other filter, is required, and the center wave-lengths and the passband widths are chosen to represent the concentrations, and algebraically combining the determined concentrations of each gas component. This might be a working method, but has the same drawbacks and problems as mentioned above in the context of U.S. Pat. No. 4,423,739.